Steel Material for Machine Structural Use Reduced in Thermal Deformation

ABSTRACT

Disclosed is a steel material with small heat treatment deformation, comprising a machine structural steel used for components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like. The steel material comprises in mass %: C: 0.16 to 0.35%; Si: 0.10 to 1.50%; Mn: 0.10 to 1.20%; P: 0 to 0.030%; S: 0 to 0.030%; Cr: 1.3 to 2.5%; Cu: 0 to 0.30%; Al: 0.008 to 0.800%; O: 0 to 0.0030%; N: 0.0020 to 0.0300%; Ni: 0 to 3.00%; Mo: 0 to 0.50%; Ti: 0 to 0.200%; Nb: 0 to 0.20%; and the balance Fe and unavoidable impurities.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2011-61209 filed on Mar. 18, 2011, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to machine structural steels, for example, used for power transmission components such as gears and shafts used in automobiles, industrial machines, and the like, and particularly relates to a machine structural steel with small heat treatment deformation.

BACKGROUND ART

Deformation of a steel material occurring due to heat treatment such as quenching (hereinafter referred to as “heat treatment deformation”) has such an adverse effect that the number of production steps is increased in order to correct the deformation, that the percentage of defective components is increased when the deformation is too large to correct, or that noise or vibration is caused by the deformation in the case where a deformed steel material is incorporated as a driving system component. Accordingly, minimizing the heat treatment deformation is a very important problem with regard to the practical use.

Conventionally, the heat treatment deformation has been considered to be affected by a large number of factors such as a component shape, the effect of a step prior to heat treatment, the physical property value of a refrigerant such as a quenching oil, and the non-uniformity of cooling as well as a steel material type, and thus reduction in heat treatment deformation by adjusting these factors properly has been attempted. For example, as a measure from the aspect of material, there has been proposed a method of precipitating a soft ferrite phase in the core of a quenched steel material to reduce heat treatment distortion (e.g., see Patent Literature 1).

As an approach from a cooling method, there has been proposed a method of utilizing pressurized gas cooling instead of conventional oil quenching (e.g., see Patent Literature 2). Further, there has been proposed a method of attempting uniform cooling of a substance to be cooled using means for promoting or decreasing a heat transfer rate (e.g., see Patent Literature 3).

In addition, in Patent Literature 3, the means for promoting a heat transfer rate is considered to be due to a coating material for promoting cooling that is placed in a site where cooling is delayed or due to convection of a coolant formed around the site where cooling is delayed, and the means for decreasing a heat transfer rate is considered to be due to glass wool or a heat insulating coating material which covers a site where cooling is easy to proceed.

CITATION LIST Patent Literature

-   [PTL 1] Japanese Patent Laid-Open Publication No. 1997-111408 -   [PTL 2] Japanese Patent Laid-Open Publication No. 2008-121064 -   [PTL 3] Japanese Patent Laid-Open Publication No. 2010-174289

SUMMARY OF THE INVENTION

However, the conventional methods proposed above have not always been versatile means in that the technology of Patent Literature 1 involves introduction of the soft phase with low strength into a component, that the technology of Patent Literature 2 requires a modification of a heat treating furnace itself, and that the technology of Patent Literature 3 requires treatment to an individual component to be heat-treated.

In contrast, the inventors have extensively researched a steel of which the heat treatment deformation can be reduced to a low level even when cooling of a component is non-uniform under a common technique such as oil quenching, while securing sufficient strength of the steel material without relying on a soft layer such as ferrite. As a result, the inventors have found that the heat treatment deformation can be reduced to a low level by suitably controlling the chemical constituents of the steel, martensitic transformation start temperature (Ms point), and hardenability as measured by a Jominy end quenching method.

It is therefore an object of the present invention to provide a steel material with small heat treatment deformation, comprising a machine structural steel used for components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like.

According to an embodiment of the present invention, there is provided a machine structural steel material with small heat treatment deformation, the steel material comprising in mass %:

-   -   C: 0.16 to 0.35%;     -   Si: 0.10 to 1.50%;     -   Mn: 0.10 to 1.20%;     -   P: 0 to 0.030%;     -   S: 0 to 0.030%;     -   Cr: 1.30 to 2.50%;     -   Cu: 0 to 0.30%;     -   Al: 0.008 to 0.800%;     -   O: 0 to 0.0030%;     -   N: 0.0020 to 0.0300%;     -   Ni: 0 to 3.00%;     -   Mo: 0 to 0.50%;     -   Ti: 0 to 0.200%;     -   Nb: 0 to 0.20%; and     -   the balance Fe and unavoidable impurities,     -   wherein the steel material has a martensitic transformation         start temperature (Ms point) of 460° C. or less;     -   a ratio (J9/J1.5) of hardness J9 at a distance of 9 mm from the         quenched end of the steel material to hardness J1.5 at a         distance of 1.5 mm from the quenched end of the steel material         is in a range of from 0.68 to 0.97, as measured by a Jominy end         quenching method; and     -   a ratio (J11/J1.5) of hardness J11 at a distance of 11 mm from         the quenched end of the steel material to hardness J1.5 at a         distance of 1.5 mm from the quenched end of the steel material         is in a range of from 0.63 to 0.94.

According to one preferred embodiment of the present invention, the above-described steel material is substantially free of Ni, Mo, Ti, and Nb, or comprises them at unavoidable impurity levels.

According to another preferred embodiment of the present invention, the above-described steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50%.

According to another preferred embodiment of the present invention, the above-described steel material comprises, in mass %, one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%.

According to another preferred embodiment of the present invention, the above-described steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50% and one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%.

DESCRIPTION OF EMBODIMENTS

The present invention is specifically explained below. Unless otherwise specified, “%” as used herein means mass %.

The machine structural steel material with small heat treatment deformation according to the present invention comprises in mass %: C: 0.16 to 0.35%; Si: 0.10 to 1.50%; Mn: 0.10 to 1.20%; P: 0 to 0.030%; S: 0 to 0.030%; Cr: 1.30 to 2.50%; Cu: 0 to 0.30%; Al: 0.008 to 0.800%; O: 0 to 0.0030%; N: 0.0020 to 0.0300%; Ni: 0 to 3.00%; Mo: 0 to 0.50%; Ti: 0 to 0.200%; Nb: 0 to 0.20%; and the balance Fe and unavoidable impurities, preferably consists essentially of these elements and unavoidable impurities, and more preferably consists of these elements and unavoidable impurities.

The steel material according to the present invention comprises C in an amount of 0.16 to 0.35%, preferably 0.20 to 0.30%, more preferably 0.22 to 0.27%. C is an element necessary for securing the strength of the steel material after quenching and tempering thereof or the strength of its core after carburizing, quenching and tempering thereof for a machine structural component, and adjustment of C content into a specified range is needed for reducing heat treatment deformation. A content of C of less than 0.16% fails to secure the strength, while that of more than 0.35% results in too large heat treatment deformation.

The steel material according to the present invention comprises Si in an amount of 0.10 to 1.50%, preferably 0.20 to 1.00%. Si is an element that is necessary for deoxidation and that is effective for imparting strength and hardenability required for steel. However, a content of Si of less than 0.10% fails to give the effects, while that of more than 1.50% results in deteriorated mechanical workability.

The steel material according to the present invention comprises Mn in an amount of 0.10 to 1.20%, preferably 0.20 to 0.80%, more preferably 0.20 to 0.55%. Mn is an element necessary for securing hardenability. However, a content of Mn of less than 0.10% fails to provide a sufficient effect for hardenability, while that of more than 1.20% results in deteriorated mechanical workability.

The steel material according to the present invention comprises P in an amount of 0 to 0.030%, typically more than 0 and not more than 0.030%. P is an unavoidable element that is incorporated from scrap, but its content of more than 0.030% results in grain boundary segregation to deteriorate characteristics such as impact strength and bending strength.

The steel material according to the present invention comprises S in an amount of 0 to 0.030%, typically more than 0 and not more than 0.030%. S is an element that improves machinability, but its content of more than 0.030% generates MnS, which is a non-metallic inclusion, to deteriorate crosswise toughness and fatigue strength.

The steel material according to the present invention comprises Ni in an amount of 0 to 3.00%, preferably 0.20 to 3.00%. Ni is an optional element that improves hardenability and toughness, and addition of 0.20% or more thereof is preferred for obtaining the effect. However, the Ni content of more than 3.00% significantly deteriorates workability and increases a cost.

The steel material according to the present invention comprises Cr in an amount of 1.30 to 2.50%, preferably 1.50 to 2.25%. Cr is an element necessary for securing hardenability. A content of Cr of less than 1.30% results in an insufficient effect for hardenability, while that of more than 2.50% results in inhibited carburization and also in deteriorated mechanical workability.

The steel material according to the present invention comprises Mo in an amount of 0 to 0.50%, preferably 0.05 to 0.50%. Mo is an optional element that improves hardenability and toughness, and addition of 0.05% or more thereof is necessary for obtaining the effect. However, a Mo content of more than 0.50% deteriorates workability.

The steel material according to the present invention comprises Cu in an amount of 0 to 0.30%, typically more than 0 and not more than 0.30%. Cu is an unavoidable element that is incorporated from scrap, but has an aging property and is effective at increasing strength. However, a content of Cu of more than 0.30% results in deteriorated hot workability.

The steel material according to the present invention comprises Al in an amount of 0.010 to 0.800%, preferably 0.014 to 0.600%. Al is an element that is used as a deoxidation material, and is bound to N to be precipitated as AlN to result in the effect of suppressing coarsening of grain size, as described below. Addition of 0.010% or more of Al is necessary for obtaining this effect. In contrast, addition of more than 0.800% of Al results in the formation of large-sized alumina-based inclusions, and deteriorates fatigue characteristics and workability.

The steel material according to the present invention comprises O in an amount of 0 to 0.0030%, typically more than 0 and not more than 0.0030%, preferably not more than 0.0020%. O is an element that is unavoidably contained in steel. However, an O content of more than 0.0030% results in the deterioration of workability and fatigue strength due to the increase of oxides.

The steel material according to the present invention comprises N in an amount of 0.0020 to 0.0300%, preferably 0.0020 to 0.0200%. N is finely precipitated as AIN and Nb nitrides in steel, and provides the effect of preventing coarsening of grain size, and addition of 0.0020% or more thereof is necessary for obtaining the effect. However, a content of more than 0.0300% results in the increase of the nitrides, and deteriorates fatigue strength and workability. Particularly in the steel that contains 0.020% or more of Ti, however, 0.0020 to 0.0100% of N is preferred for avoiding the deterioration of fatigue strength due to the excessive generation of TiN.

The steel material according to the present invention comprises Ti in an amount of 0 to 0.200%, preferably 0.020 to 0.200%. Ti is an optional element that is bound to C in steel to finely form a carbide, and provides the effect of preventing coarsening of grain size, and addition of 0.020% or more of Ti is preferred for obtaining the effect. In contrast, addition of more than 0.200% thereof results in deteriorated mechanical workability.

The steel material according to the present invention comprises Nb in an amount of 0 to 0.20%, preferably 0.02 to 0.20%, more preferably 0.02 to 0.12%. Nb forms a carbide or a nitride, and provides the effect of preventing coarsening of grain size. In particular, NbC or Nb(C, N) with a nanometer-order size, which is finely dispersed in steel, suppresses the growth of the grain size. Less than 0.02% of Nb fails to provide the effect, while more than 0.20% thereof results in the excessive amount of a precipitate to deteriorate workability.

In the steel material according to the present invention, its martensitic transformation start temperature (Ms point) is regulated to 460° C. or less, preferably 450° C. or less, in order to reduce the heat treatment deformation of the steel material. The reason why the heat treatment deformation can be reduced by regulating the Ms point to 460° C. or less is that occurrence of martensitic transformation during quenching can be avoided in a temperature range in which the cooling performance of a refrigerant is high, even when cooling of a component is non-uniform, and, as a result, a time point at which the martensitic transformation occurs can be inhibited from greatly differing depending on the site of the component. The heat treatment deformation in this case refers to bending of a shaft-shaped component or snapping or twisting of a gear tooth.

In the steel material according to the present invention, a ratio (J9/J1.5) of hardness J9 at a distance of 9 mm from the quenched end of the steel material to hardness J1.5 at a distance of 1.5 mm from the quenched end of the steel material, as measured by a Jominy end quenching method, is in a range of from 0.68 to 0.97; and a ratio (J11/J1.5) of hardness J11 at a distance of 11 mm from the quenched end of the steel material to hardness J1.5 at a distance of 1.5 mm from the quenched end of the steel material is in a range of from 0.63 to 0.94. Within such a range, the heat treatment deformation of the steel material can be suppressed to a low level. The heat treatment deformation in this case refers to a variation in dimension (length, diameter, or thickness) before and after quenching a component. Although a mechanism to suppress the heat treatment deformation by suitably controlling Jominy hardenability has not yet been elucidated, it has been experimentally revealed by the inventors that the heat treatment deformation becomes large when the hardenability of a steel material is too low or too high.

The limitation of the steel constituents, the limitation of the Ms point, and the limitation of hardenability measured by the Jominy end quenching method, as mentioned above, achieves the smaller heat treatment deformation in the case of processing a steel material into a component and then performing quenching or carburizing and quenching for hardening the component. As a result, the present invention can provide such a beneficial effect of making it possible to improve the yield of components, to simplify or remove the step of correcting a component, or to omit the grinding of a gear tooth surface for measures against noise and vibration.

EXAMPLES

The steel material according to the present invention is specifically explained with reference to Examples below. As the machine structural steel used as components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like, machine structural steels comprising compositions of present invention examples Nos. 1 to 23 shown in Table 1 and the balance Fe and unavoidable impurities were ingotted in a vacuum induction melting furnace to obtain 100 kg of steel ingots.

TABLE 1 (unit: mass %) No. C Si Mn P S Ni Cr Mo Cu Al O N Others Invention 1 0.16 0.15 0.88 0.009 0.010 0.06 1.72 0.01 0.11 0.035 0.0009 0.0152 — Examples 2 0.22 0.52 0.25 0.012 0.005 0.04 1.86 0.02 0.05 0.055 0.0011 0.0188 — 3 0.24 0.52 0.66 0.013 0.008 0.11 1.35 0.01 0.06 0.025 0.0008 0.0160 — 4 0.27 0.25 1.02 0.007 0.012 0.05 1.60 0.02 0.05 0.018 0.0007 0.0134 — 5 0.29 0.20 0.32 0.010 0.006 0.12 1.55 0.02 0.05 0.102 0.0005 0.0140 — 6 0.33 0.80 0.48 0.020 0.014 0.10 1.65 0.01 0.06 0.032 0.0007 0.0155 — 7 0.20 0.12 0.43 0.012 0.017 0.55 2.23 0.02 0.11 0.035 0.0011 0.0125 — 8 0.18 0.25 0.51 0.015 0.013 0.80 1.34 0.02 0.10 0.051 0.0012 0.0155 — 9 0.24 0.20 0.42 0.006 0.007 1.15 1.42 0.01 0.09 0.045 0.0008 0.0165 — 10 0.26 0.52 0.45 0.005 0.007 0.11 1.50 0.06 0.09 0.029 0.0008 0.0154 — 11 0.27 0.65 0.33 0.010 0.011 0.05 1.35 0.15 0.09 0.030 0.0008 0.0132 — 12 0.33 0.17 0.18 0.013 0.009 0.06 1.51 0.25 0.11 0.016 0.0009 0.0128 — 13 0.23 0.31 0.62 0.008 0.008 1.07 1.32 0.30 0.04 0.020 0.0007 0.0118 — 14 0.17 0.21 0.45 0.010 0.010 0.07 2.32 0.01 0.03 0.032 0.0012 0.0151  0.03% Nb 15 0.23 0.20 0.31 0.012 0.013 0.12 1.88 0.01 0.11 0.034 0.0010 0.0120  0.06% Nb 16 0.22 0.30 0.35 0.011 0.006 0.08 1.84 0.02 0.02 0.056 0.0012 0.0070 0.034% Ti  0.05% Nb 17 0.25 0.54 0.42 0.008 0.008 0.02 1.91 0.01 0.11 0.042 0.0015 0.0034 0.102% Ti 18 0.29 0.58 0.82 0.007 0.008 0.02 1.35 0.02 0.11 0.021 0.0007 0.0060 0.166% Ti 19 0.21 0.62 0.33 0.008 0.015 0.40 1.55 0.01 0.12 0.023 0.0009 0.0200  0.03% Nb 20 0.22 0.34 0.72 0.012 0.012 0.90 1.42 0.02 0.13 0.017 0.0004 0.0182  0.04% Nb 21 0.24 0.53 0.28 0.014 0.013 2.10 1.62 0.02 0.08 0.018 0.0005 0.0054 0.025% Ti  0.07% Nb 22 0.26 0.60 0.25 0.006 0.009 0.03 1.91 0.09 0.06 0.033 0.0006 0.0065 0.120% Ti 23 0.32 0.85 0.20 0.008 0.007 0.04 1.32 0.25 0.05 0.040 0.0009 0.0028 0.190% Ti (0.20% or less Ni and 0.05% or less Mo are unavoidable impurities.)

In the same manner as in the present invention examples described above, as the machine structural steel used for components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like, steels comprising compositions of comparative examples Nos. 1 to 16 shown in Table 2 and the balance Fe and unavoidable impurities were ingotted in the vacuum induction melting furnace to obtain 100 kg of steel ingots.

TABLE 2

(The shaded portions fall outside the scope of claims. 0.20% or less Ni and 0.05% or less Mo are unavoidable impurities.) (unit: mass %)

First, the steel ingots of the present invention examples and the comparative examples were heated at 1250° C. for 5 hours and then forged to obtain steel bars having a diameter of 32 mm. Then, the steel bars were normalized by heating and maintaining at 900° C. for 1.5 hours, followed by air-cooling. Subsequently, test pieces having a diameter of 20 mm and a length of 200 mm were produced from the steel bars having the diameter of 32 mm, and the sides of the test pieces were subjected to processing to have a groove with a depth of 5 mm, a width of 8 mm, and a length of 200 mm. The groove processing caused a cooling rate to greatly differ depending on a site in each test piece during quenching. Further, the lengths of the test pieces after the groove processing were measured. Subsequently, the test pieces were heated at 930° C. for 1 hour, their temperatures were then decreased to 850° C. in the furnace, and were further maintained for 1 hour and then quenched in a quenching oil at 60° C. After the quenching, as for the sufficiently cooled test pieces, the bends and lengths of the test pieces were measured.

Each bend after the heat treatment was determined by holding both ends of each test piece with V blocks, measuring the maximum and minimum displacements on the circumference of the central portion of the test piece during one revolution of the test piece, with a dial gauge, and dividing the difference between the maximum and minimum displacements by 2. In the case of the measurement, the displacement at the bottom of a groove present on the circumference of the test piece was ignored. Further, the difference between the lengths of the test piece before and after the heat treatment was determined to evaluate the absolute value thereof as the index of dimensional change.

Further, test pieces having a diameter of 3 mm and a length of 10 mm were extracted from the steel bars having a diameter of 32 mm after subjected to the normalizing described above to measure Ms points as the martensitic transformation start temperatures of the steel materials using a fully-automated transformation record measuring apparatus. The Ms points in this context are measured under the conditions simulating the cooling process of a component. In the present embodiment, the Ms points were measured at a cooling rate of 30° C./s during the quenching by simulating the case where the above-described test pieces with grooves having the diameter of 20 mm were oil quenched at an oil temperature of 60° C. For the measurement of the hardenability of the steel materials by the Jominy end quenching method, test pieces were produced from the above-described forged steel bars having the diameter of 32 mm, and were tested and evaluated under the conditions according to “method of hardenability test for steel” (end quenching method) specified in JIS G 0561.

In Table 3, there are shown the measured Ms points of the present invention examples, each value of hardness J1.5 at a distance of 1.5 mm from the quenched ends, hardness J9 at a distance of 9 mm, and hardness J11 at a distance of 11 mm, measured by the Jominy end quenching method, and the determined values of (J9/J1.5) and (J11/J1.5). The bends (mm) evaluated after quenching the above-described test pieces and the absolute values (mm) of the differences between the lengths of the test pieces before and after the heat treatment are further shown. In the steel materials of the present invention examples Nos. 1 to 23, as shown in Table 3, the martensitic transformation start temperatures, i.e., the Ms points were in a range of 372 to 442° C., the values of equation (1), shown as follows, of (J9/J1.5) of the steel materials were in a range of 0.70 to 0.95, the values of equation (2), shown as follows, of (J11/J1.5) were in a range of 0.65 to 0.90, the bends after the heat treatment were 0.10 to 0.36 mm, and the absolute values of the differences between the lengths of the test pieces before and after the heat treatment were 0.01 to 0.20 mm.

(J9/J1.5)=(hardness at distance of 9 mm from quenched end, measured by Jominy end quenching method)/(hardness at distance of 1.5 mm from quenched end, measured by Jominy end quenching method)  (1)

(J11/J1.5)=(hardness at distance of 11 mm from quenched end, measured by Jominy end quenching method)/(hardness at distance of 1.5 mm from quenched end, measured by Jominy end quenching method)  (2)

TABLE 3 |Length after Heat Treatment − Ms Length before Point J1.5 J9 J11 Bend Heat Treatment| No. (° C.) (HRC) (HRC) (HRC) (J9/J1.5) (J11/J1.5) (mm) (mm) Invention 1 433 42.3 33.3 30.8 0.79 0.73 0.31 0.05 Examples 2 441 46.1 32.2 30.5 0.70 0.66 0.26 0.17 3 421 48.0 36.5 33.7 0.76 0.70 0.23 0.11 4 375 49.3 44.1 41.8 0.89 0.85 0.16 0.02 5 413 50.2 37.7 34.6 0.75 0.69 0.24 0.08 6 372 53.0 45.7 42.8 0.86 0.81 0.19 0.04 7 420 44.3 35.3 33.5 0.80 0.76 0.31 0.03 8 440 43.1 34.6 32.2 0.80 0.75 0.36 0.06 9 405 47.3 35.1 32.6 0.74 0.69 0.14 0.13 10 411 48.6 36.9 34.5 0.76 0.71 0.25 0.11 11 424 49.5 37.4 35.0 0.76 0.71 0.28 0.09 12 389 52.7 39.3 36.9 0.75 0.70 0.10 0.08 13 379 46.5 44.4 42.1 0.95 0.90 0.17 0.20 14 442 42.8 33.1 30.6 0.77 0.72 0.28 0.13 15 423 46.4 32.4 29.9 0.70 0.65 0.15 0.14 16 430 47.2 37.1 34.7 0.79 0.73 0.30 0.07 17 420 48.8 39.2 36.8 0.80 0.75 0.31 0.01 18 385 50.3 42.8 40.4 0.85 0.80 0.15 0.06 19 442 45.2 32.1 29.6 0.71 0.66 0.28 0.10 20 400 46.8 37.9 35.5 0.81 0.76 0.26 0.12 21 375 47.6 39.5 37.1 0.83 0.78 0.20 0.08 22 415 49.3 38.2 35.8 0.78 0.73 0.23 0.05 23 405 52.5 41.9 39.6 0.80 0.75 0.33 0.12

Similarly, in Table 4, there are shown the measured Ms points of the steels of the comparative examples, Jominy hardenability as hardness J1.5 at a distance of 1.5 mm from the quenched ends, hardness J9 at a distance of 9 mm, or hardness J11 at a distance of 11 mm, measured by the Jominy end quenching method, and the determined values of (J9/J1.5) and (J11/J1.5). The bends (mm) after quenching of the above-described test pieces and the absolute values (mm) of the differences between the lengths of the test pieces before and after the heat treatment are further shown.

TABLE 4

(The shaded portions fall outside the scope of claims.)

In the above-described invention examples Nos. 1 to 23, the bends of the test pieces after the heat treatment were able to be reduced to the low range of 0.10 to 0.36 mm and the absolute values of the differences between the lengths of the test pieces before and after the heat treatment were also able to be reduced to the small range of 0.01 to 0.20 mm by formulating the composition ranges of the steel materials, except for Fe and unavoidable impurities other than Ni and Mo, to those shown in Table 1, by setting the Ms points at 372 to 442° C., which were lower than or equal to 460° C., and by suitably controlling hardenability measured by the Jominy end quenching method, thereby setting the values of (J9/J1.5) calculated by equation (1) in a range of 0.70 to 0.95 and setting the values of (J11/J1.5) calculated by equation (2) in a range of 0.65 to 0.90.

In contrast, in the above-described comparative examples Nos. 1 to 16, in which the composition ranges of the steel materials, except for Fe and unavoidable impurities other than Ni and Mo, are shown in Table 2, the remaining 14 examples excluding two examples No. 2 and No. 16 had the composition ranges falling outside those of the present invention. Among the steels of the comparative examples Nos. 1 to 16, ten examples of which the measured Ms points are higher than 460° C. have bends of the test pieces after the heat treatment of 0.49 to 0.76 mm, which are greater than those of the steels of the invention examples. Further, among the comparative steels of Nos. 1 to 16, 15 examples, in which (J9/J1.5) determined by equation (1) and equation (2) from hardness as measured by the Jominy end quenching method falls outside the range of from 0.68 to 0.97 and the (J11/J1.5) values fall outside the range of from 0.63 to 0.94, have absolute values of the differences between the lengths of the test pieces before and after the heat treatment of 0.27 to 0.45 mm, which are greater than those of the steels of the invention examples. Accordingly, none of the comparative examples had both of the bend of the test piece after the heat treatment and the absolute value of the difference between the lengths of the test piece before and after the heat treatment that were equivalent to those of the steels of the present invention examples.

In comparison with the comparative examples Nos. 1 to 3, 5 to 7, 9, 10, 14, and 15 of which the measured Ms points fall outside the scope of claims, the present invention examples Nos. 1 to 23 of which the Ms points satisfy the scope of claims have low bends of the test pieces after the heat treatment, resulting in reduced heat treatment deformation. Further, in comparison with the comparative examples Nos. 1 and 3 to 16 of which the values of (J9/J1.5) and the values of (J11/J1.5) fall outside the scope of claims, the invention examples Nos. 1 to 23 of which the values of (J9/J1.5) and the values of (J11/J1.5) satisfy the scope of claims have the low absolute values of the differences between the lengths of the test pieces before and after the heat treatment, resulting in reduced heat treatment deformation. Although the heat treatment deformation is evaluated by quenching instead of carburizing and quenching in the present embodiment, it is confirmed that the heat treatment deformation of the steel material that satisfies the present claims is small even in the case of carburizing and quenching. In addition, the steel materials of the present invention examples are quenched, and then tempered before use.

In view of the above, smaller heat treatment deformation after processing a steel material into a component and then performing quenching or carburizing and quenching for hardening the component can be achieved by the limitation of the steel constituents, the limitation of an Ms point as martensitic transformation start temperature, and the limitation of hardenability measured by the Jominy end quenching method, in the present invention. Such a steel material according to the present invention is a steel material that can be applied to components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like. 

1. A machine structural steel material with small heat treatment deformation, the steel material comprising in mass %: C: 0.16 to 0.35%; Si: 0.10 to 1.50%; Mn: 0.10 to 1.20%; P: 0 to 0.030%; S: 0 to 0.030%; Cr: 1.30 to 2.50%; Cu: 0 to 0.30%; Al: 0.008 to 0.800%; O: 0 to 0.0030%; N: 0.0020 to 0.0300%; Ni: 0 to 3.00%; Mo: 0 to 0.50%; Ti: 0 to 0.200%; Nb: 0 to 0.20%; and the balance Fe and unavoidable impurities, wherein the steel material has a martensitic transformation start temperature (Ms point) of 460° C. or less; a ratio (J9/J1.5) of hardness J9 at a distance of 9 mm from the quenched end of the steel material to hardness J1.5 at a distance of 1.5 mm from the quenched end of the steel material is in a range of from 0.68 to 0.97, as measured by a Jominy end quenching method; and a ratio (J11/J1.5) of hardness J11 at a distance of 11 mm from the quenched end of the steel material to hardness J1.5 at a distance of 1.5 mm from the quenched end of the steel material is in a range of from 0.63 to 0.94.
 2. The machine structural steel material according to claim 1, wherein the steel material consists of in mass %: C: 0.16 to 0.35%; Si: 0.10 to 1.50%; Mn: 0.10 to 1.20%; P: 0 to 0.030%; S: 0 to 0.030%; Cr: 1.30 to 2.50%; Cu: 0 to 0.30%; Al: 0.008 to 0.800%; O: 0 to 0.0030%; N: 0.0020 to 0.0300%; Ni: 0 to 3.00%; Mo: 0 to 0.50%; Ti: 0 to 0.200%; Nb: 0 to 0.20%; and the balance Fe and unavoidable impurities.
 3. The machine structural steel material according to claim 1, wherein the steel material is substantially free of Ni, Mo, Ti, and Nb.
 4. The machine structural steel material according to claim 2, wherein the steel material is substantially free of Ni, Mo, Ti, and Nb.
 5. The machine structural steel material according to claim 1, wherein the steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50%.
 6. The machine structural steel material according to claim 2, wherein the steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50%.
 7. The machine structural steel material according to claim 1, wherein the steel material comprises, in mass %, one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%.
 8. The machine structural steel material according to claim 2, wherein the steel material comprises, in mass %, one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%.
 9. The machine structural steel material according to claim 1, wherein the steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50% and one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%.
 10. The machine structural steel material according to claim 2, wherein the steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50% and one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%. 